Currently, in the semiconductor industry there is a great interest in the characterization of thin films and small structures. Integrated circuits are made up of a large number of patterned thin films deposited onto a semiconductor substrate, such as silicon. The thin films include metals to make connections between the transistors making up the chip, and insulating films to provide insulation between the metal layers (see: S. A. Campbell, The Science and Engineering of Microelectronic Fabrication, Oxford University Press, (1996)). The metal films (interconnects) are typically arranged as a series of patterned layers. At the present time there may be 4 or 5 layers of interconnects. It is likely that as more complex integrated circuits are developed, requiring a greater number of interconnections, the number of layers will increase. Metals of current interest include, for example, aluminum, copper, titanium and silicides. Insulating films include, for example, oxide glasses of various compositions and polymers. The films may be patterned so as to form wires running across the surface of the sample. For convenience, each such wire shall be referred to as a structure. These wires may be embedded into a film of another material or may be deposited on top of another film. For some samples of interest, all of the wires have the same nominal dimensions, run in the same direction across the surface of the sample, and are equally spaced. If the wires run in the direction parallel to the z-axis, for example, the geometry of the sample is entirely specified when the cross-section in the x-y plane is determined (see FIG. 1). For this reason such samples are referred to as two-dimensional patterned structures. Another type of sample of interest might include a two-dimensional array of identical rectangular parallelepipeds disposed on a surface (see FIG. 2). The geometry of such a sample cannot be completely specified by determining the geometry in a single x-y plane. For this reason, a sample of this type is referred to as a three-dimensional patterned sample. Other samples might still be periodic, but with a more complicated pattern. For example, the sample could be made up of a sequence ABABAB, of wires with two different dimensions, such that wire A has width aA and height bA, and wire B has width aB and height bB. Alternately, the sample could include a sequence of wires, all of which have the same geometry, but the spacing between the wires could alternate between the values c1 and c2.
In the production of integrated circuits it is essential that all aspects of the fabrication process be controlled as closely as possible. It is important to measure the geometry of the sample, i.e., the thickness of thin films, the lateral dimensions of wire structures such as the dimensions a, and b in FIG. 1, the spacing c between structures, etc. It is also important to be able to measure mechanical and electrical properties of the structure, such as the adhesion between a wire and the film it is in contact with.
There are a number of techniques currently available for the determination of the geometry of such samples. These include:
(1) Scanning Electron Microscopy. In this technique an electron beam is focused onto a small spot on the sample, and electrons that are scattered from the sample surface are detected. The electron beam is scanned across the surface of the sample, and an image of the sample surface is obtained. For a two-dimensionally patterned sample this technique can determine the dimensions a and c as shown in FIG. 1. For a three-dimensionally patterned sample the dimensions a1, a2, c1, and c2 of FIG. 2 can be determined. This method cannot be used to determine the dimension b of FIG. 1. In addition, the method is time consuming since the sample must be placed into the high vacuum chamber of the electron microscope. In addition, to measure dimensions with scanning electron microscopy it is necessary to perform a careful calibration of the instrument.
(2) Scanning Electron Microscopy with Sectioning. In this technique, material is removed from the sample to expose a section of the sample lying in the xy-plane. Scanning electron microscopy is then used to view this section of the sample. This method is thus able to measure the dimension b shown in FIGS. 1 and 2. This method has the following disadvantages: i) A considerable amount of time is required to prepare the sample. ii) The sample has to be destroyed in order to make the measurement. iii) The method is time-consuming since the sample has to be transferred into the high-vacuum chamber of the electron microscope in order for the measurement to be made.
(3) Atomic Force Microscopy. In this technique an atomic force microscope is used instead of an electron microscope to view the surface of the sample. The top surface of the sample can be viewed directly, as in (1) above, and measurements can also be made after sectioning the sample as in method (2). This, method has the disadvantage that a considerable amount of time is involved for the measurements to be made. In addition, if the sample is sectioned, it is destroyed.